OSPFv2 and OSPFv3 headers

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All OSPFv2 packets have a common 24-byte header, and OSPFv3 packets have a common 16-byte header, that contains all information necessary to determine whether OSPF should accept the packet. The header consists of the following fields:

• Version number—The current OSPF version number. This can be either 2 or 3.
• Type—Type of OSPF packet.
• Packet length—Length of the packet, in bytes, including the header.
• Router ID—IP address of the router from which the packet originated.
• Area ID—Identifier of the area in which the packet is traveling. Each OSPF packet is associated with a single area. Packets traveling over a virtual link are labeled with the backbone area ID, 0.0.0.0. .
• Checksum—Fletcher checksum.
• Authentication—(OSPFv2 only) Authentication scheme and authentication information.
• Instance ID—(OSPFv3 only) Identifier used when there are multiple OSPFv3 realms configured on a link.